Add Leaflet marker cluster plugin

Just added the Leaflet marker cluster plugin.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Fix Leaflet map loading

The Leaflet map did not load properly when the map tabular is activated.
This was due to the map being initialized while the actual HTML element
is not visible. Now, the map is not initialized until the element is
visible.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>